Flames and burners

Flames make use of propane, acetylene and hydrogen as the burning gases together with air or nitrous oxide as the oxidant gas. With these mixtures, temperatures between 2000 and 3000 K can be obtained as listed below  

For more than 30 elements, however, the temperature of this flame is too low. This applies to elements that have very stable oxides such as V, Ti, Zr, Ta and the lanthanides. These elements require the use of a nitrous oxide-acetylene flame, as was introduced into flame AAS by Amos and Willis in 1966 [264], which is more suitable. However, this flame has a higher background emission, due in particular to the radiation of the CN bands. 

The hydrogen-air flame is of little use. For the determination of Sn, however, its sensitivity is higher than the acetylene-air flame. Because of its higher transmission at short UV wavelengths the hydrogen-air flame is also used for the determination of As and Se with the 193.7 nm and the 196.0 nm lines, respectively. 

The propane-air flame has the lowest temperature and finds use for the determination of easily atomized elements such as Cd, Cu, Pb, Ag, Zn and specifically the alkali elements. 

It should be kept in mind that the solvent used may have a considerable influence on the flame temperature. The influence of the sample matrix on the flame temperature is low and therefore its consequences for the degree of dissociation are negligible. The equilibrium itself is only a function of the temperature but the speed with which the equilibrium is reached depends on the relevant reaction mechanisms and the reaction velocities. 

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It is worth stressing that everything that has ever been published on the application of AAS to food analysis can be done at least as well with HR-CS AAS. Since the same flames and burners, and the same type of electrothermal atomizers, are used in both systems, and only the spectrometer part from the radiation source to the detector has been re-designed, it is much more appropriate to talk about the improvements brought about by this change, and about the simplifications and the additional features that have become available this way. 

Regarding historical insight and descriptions of principles and fundamentals of flame atomic emission spectrometry, a chapter on flame photometry appeared in the first edition of Treatise on Analytical Chemistry (Vallee and Thiers 1965) covering the flame and burner, photometer/spec-trometer, fundamental discussion of excitation and processes within the flame, cation and anion interferences and handling of analytical samples. In an analogous, expanded, detailed and excellent treatment of EAES in the second edition of the Treatise on Analytical Chemistry, Syty (1981) discusses types of flames used for excitation, processes within flames, spectral, chemical and physical interferences and remedies. 

Clogging the aspirator and burner assembly decreases the rate of aspiration, decreasing the analyte s concentration in the flame. The result is a decrease in the signal and the introduction of a determinate error. 

Fig. 3. Flame hardening (a) tempeiatuie—time heating curves of a 25 x 50 x 100 mm specimen at a feed of 75 mm /min and burner distance of 8 mm showing temperatures of A, surface B, 2 mm below surface and C, 10 mm below surface (b) hardness—depth curves for a 0.50% C steel 25 X 75 X 100 mm specimen at a feed of 50 mm /min, temperatures ia °C measured 10 mm below the surface, and burner distances ia mm, respectively, of A, 530 and 50 B, 540 and 12 C, 545 and 10 D, 550 and 8 and E, 565 and 6. Flame heating followed by water spray quenching. HV = Vickers hardness.

When used for ceramic heating, furnaces are called Idlus. Operations include drying, oxidation, c cination, and vitrification. These Idlus employ horizontal space burners with gaseous, hquid, or solid fuels. If product quahty is not injured, ceramic ware may be exposed to flame and combustion gases otherwise, muffle Idlus are employed. Dutch ovens are used frequently for heat generation. 

Equation (12-57) does not account for gas radiation at high temperature when the kiln charge can see the burner flame hence, the method will yield a conservative design. When a kiln is fired internally, the major source of heat transfer is radiation from the flame and hot gases. This occurs directly to both the sohds surface and the wall, and from the latter to the product by reradiation (with some conduction). 

A horizontally fired burner is located at one end of the heater. The flame extends along the central longitudinal axis of the heater. In this way the wickets are exposed to the open flame and can be subjected to a maximum rate of radiant heat transfer. The tubes should be sufficiently far away from the flame to prevent hot spots or flame pinching. 

Exhaust sparks from engines and burners can be a source of ignition, Any open flame on the facility can also be a source of ignition. 

Caution Addition of 0 5 cm pieces of sodium metal to methanol or ethanol must be done in a chemical hood and behind a safety shield. Addition should be slow to minimize evaporation loss of methanol or ethanol. No flames or burner should be permitted in the area. Disposal of sodium metal must be earned out in someone s presence. 

The total consumption type of burner consists of three concentric tubes as shown in Fig. 21.5. The sample solution is carried by a fine capillary tube A directly into the flame. The fuel gas and the oxidant gas are carried along separate tubes so that they only mix at the tip of the burner. Since all the liquid sample which is aspirated by the capillary tube reaches the flame, it would appear that this type of burner should be more efficient that the pre-mix type of burner. However, the total consumption burner gives a flame of relatively short path length, and hence such burners are predominantly used for flame emission studies. This type of burner has the advantages that (1) it is simple to manufacture, (2) it allows a totally representative sample to reach the flame, and (3) it is free from explosion hazards arising from unbumt gas mixtures. Its disadvantages are that (1) the aspiration rate varies with different solvents, and (2) there is a tendency for incrustations to form at the tip of the burner which can lead to variations in the signal recorded. 

Pulverized fuel coal burners (typically turbulent air burners, vertical burners, or nozzle burners) receive hot primary air containing the PF and introduce the mixture to secondary air in such a way that it provides a stable flame. The flow rates of both primary and secondary air are controlled by dampers. An ignitor is required to initiate combustion, and the flame front is maintained close to the burner, with the heat of combustion used to ignite incoming PF. A flame safety device electronically scans the flame and initiates corrective action if required. 

Under low-frequency excitation, the flame front is wrinkled by velocity modulations (Fig. 5.2.5). The number of undulations is directly linked to frequency. This is true as far as the frequency remains low (in this experiment, between 30 and 400 Hz). The flame deformation is created by hydrodynamic perturbations initiated at the base of the flame and convected along the front. When the velocity modulation amplitude is low, the undulations are sinusoidal and weakly damped as they proceed to the top of the flame. When the modulation amplitude is augmented, a toroidal vortex is generated at the burner outlet and the flame front rolls over the vortex near the burner base. Consumption is fast enough to suppress further winding by the structure as it is convected away from the outlet. This yields a cusp formed toward burnt gases. This process requires some duration and it is obtained when the flame extends over a sufficient axial distance. If the acoustic modulation level remain low (typically v /v < 20%),... 

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